1. Field of the Invention
The present invention relates to an image acquisition apparatus.
2. Description of the Related Art
In the technical field of image acquisition, in a solid-state imaging element such as a CCD, generally, the respective imaging pixels are arranged in a square lattice form. Further, the output image signal from a camera formed by using such imaging elements is converted into and used in an appropriate time-series signal format in accordance with the application. For example, if the camera is a camera for movies, the image signal is output by an analog video format such as NTSC or S-VHS or the like, and is displayed on a monitor. If the camera is a digital still camera, the image signal is converted into a digital signal and is recorded onto a medium such as an IC card or the like. However, the signal processing in such processes is limited to format conversion processing or inter-polation processing, or to color conversion processing, edge enhancing processing or the like for improving the appearance. The obtained image is structured in direct accordance with the arrangement of the imaging pixels.
Namely, in the above-described ordinary imaging/display system, assuming that the display device such as a monitor is structured by discrete display pixels, it can be thought that the imaging pixels and the display pixels are both arranged in square lattice forms and are in a one-to-one correspondence. Note that, in the present specification, even when different sampling intervals are set for two orthogonal directions in a plane, for the convenience of explanation, it is called a square lattice. If an attempt is made to carry out transfer or recording efficiently in such a system, a digital image data compression technique, e.g., the MPEG standard for movies or the JPEG standard for static images, is used. In other words, the arrangement of the pixels and the number of the pixels of the image acquisition section and the image output section are made equal, and carrying out compression conversion at a transfer/recording section along the way can be considered.
In contrast, examples have been proposed in which the imaging pixels are disposed uniquely, and not in a square lattice form. As one such conventional example, a structure of a digital still camera using imaging elements structured by a pixel shifted arrangement (a honeycomb arrangement) is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 11-168688. This publication discloses a method in which the number of pixels is increased through interpolation processing on an input signal by imaging pixels in a honeycomb arrangement, and a square lattice arrangement is formed. In accordance with this method, an image appearing to be more highly detailed when viewed can be generated with a limited number of pixels, and the predominance of the imaging section in the structure is emphasized. Further, although the JPEG compression technique is employed in this camera as well, attention is focused on the point that the arrangement of the pixels and the number of the pixels are fundamentally different at the image acquisition section and the image output section.
Conventional solid-state imaging elements are generally structured by imaging pixels arranged in a square lattice form. It can be thought that, in an imaging/display system used by a camera using such imaging elements, the imaging pixel arrangement and the display pixel arrangement are substantially in a one-to-one correspondence. However, in such a system, if there are any constraints on the imaging section, e.g., even if it is necessary to form a camera whose usable space is extremely small, a pixel structure which is equivalent to that of the display section is required for the imaging section.
Further, in a case where the capacity of the signal is limited because the transfer band of the image signal is narrow, when a conventional digital image data compression method must be employed, an image structure of a sufficiently large scale is required regardless of the imaging elements, and further, a processor which realizes digital compressing functions must be loaded at the imaging section. With such a system, it is extremely difficult to realize, for example, a system monitoring detailed functions in industrial facilities, visual functions of microrobots, extremely minute endoscopes used in medical diagnosis, or the like.
On the other hand, a camera utilizing a honeycomb arrangement is a conventional example in which constraints on the imaging section are eased due to the pixel arrangement of the imaging elements being freed from the square lattice. However, in this example as well, there is ultimately no change in imaging by a uniform sampling rate within the imaging surface, and the number of pixels which can be eliminated is at most half.
Moreover, because the image restoration processing is simple interpolation processing, it is only applicable to cases where the imaging pixels are arranged uniformly and systematically. Namely, no method has been proposed which, even when, due to some constraint, only a lens having inferior performances can be loaded and the spatial frequency transfer characteristics differ greatly due to the position within the imaging surface, is suited for such conditions, and also can carry out efficient imaging by arranging pixels irregularly. In addition, there has been no clear disclosure of an interpolation method suited for irregular pixel arrangements.
In this way, there has not been proposed a method which improves the degrees of freedom in design of the imaging section under strict constraints, by completely correlating the imaging optical system and the imaging pixel arrangement and the restoration processing in a conventional imaging/display system using solid-state imaging elements.